1. Field of the Invention
The present invention relates to a light guide plate used in a backlight module for a liquid crystal display, and more particularly to a light guide plate having a dot pattern used in a backlight module for a liquid crystal display.
2. Description of Prior Art
Generally, a liquid crystal display device includes a liquid crystal panel having two substrates and a liquid crystal inserted therebetween, and a backlight module disposed under the liquid crystal panel. The backlight module is arranged to distribute light from a light source uniformly over the surface of the liquid crystal panel. There are several kinds of backlight modules, including a direct back light type (or direct type) and an edge light type. Generally, in a direct type backlight module, a light source is disposed inside the display area of the liquid crystal panel, thus, the brightness distribution of the liquid crystal panel is not uniform and the liquid crystal and the TFT switching elements can be damaged by the heat energy of the light source. Furthermore, it is very difficult to reduce the thickness of the direct type backlight module.
Generally, an edge type backlight module comprises a light guide plate, a light source attached to at least one edge of the light guide plate, and a reflecting sheet formed at a bottom surface of the light guide plate, a diffusing sheet disposed on the light guide plate and a light condenser disposed on the diffusing sheet. The light source can be a linear light source or a point light source. Because the light source is disposed at the edge of the light guide plate, the thickness of the LCD can be minimized. Furthermore, the liquid crystal and the TFT switching elements are not damaged by the heat energy of the light source. Therefore, the edge light type has some advantages over the direct type in that it is thinner and provides enhanced picture quality.
However, the brightness of the light emitted to the liquid crystal panel by the edge type backlight module decreases according to the distance from the light source. In order to overcome this problem, and to improve the uniformity and efficiency of luminance, a dot pattern or a pattern of V-shaped grooves is formed on the bottom surface of the light guide plate such that a density of the dot pattern or the V-shaped groove pattern varies as a function of distance from the light source. Using these patterns in a light guide plate results in the brightness of the light which is incident on the liquid crystal panel being made more uniform. In the case of the dot pattern, a size of the dots increases as the distance from the light source attached on one edge of the light guide plane increases. In the case of the V-shaped groove pattern, the distance between adjacent grooves decreases as the distance from the light source increases. When the incident light transmits in the light guide plate, the dot pattern or the V-shaped groove pattern scatters the incident light for uniformly transmitting the light to the light output surface.
U.S. Pat. No. 5,363,294 discloses a surface light source device having a light guide plate with a dot pattern (shown in FIG. 4). The surface light source device comprises a light guide plate 1, a linear light source 2 positioned along one end edge of the light guide plate 1, an end edge reflection layer 4 provided on an opposite end edge of the light guide plate 1, a light diffusion/transmission section 3 provided on a back face of the light guide plate 1, a diffusion layer 6 provided on an emitting face opposite to the back face of the light guide plate 1, a back face reflection layer 5 provided on the back face of the light guide plate 1, and a curved reflection plate 7 provided to enclose the linear light source 2. The light diffusion/transmission section 3 has a dot pattern.
Further referring to FIG. 5, a ratio of an area of the light diffusion/transmission section 3 which is covered by the dots as a portion of the whole area of the back face of the light guide plate 1 gradually increases with an increase in distance from the light source 2, and then is constant in a large, furthest end region from the light source 2.
However, light generated by the light source 2 and transmitted in the light guide plate 1 generally reflects several times between the emitting face and the back face of the light guide plate 1. A decrease in light intensity emitted at the emitting face is not linear with an increase in distance from the light source 2. Thus, it is difficult to achieve an uniformity in light output from the emitting face since the ratio of the area of the dots as a function of total area increases linearly from the light source to the region near the end edge of reflection layer 4.